This invention relates to a phase shift circuit and, in particular, to a phase shift circuit having an adjustable phase shift over a wide range of frequencies.
An all-pass filter is a filter that operates in the time domain rather than in the frequency domain. The filter passes all frequencies without affecting amplitude but shifts phase in proportion to frequency. Such filters are described in detail in chapter 7 of "Electronic Filter Design Handbook," A. B. Williams and F. J. Taylor, 3rd Ed., McGraw-Hill, Inc. 1995. The Williams et al. text is the only text known that describes all-pass filters in detail.
At high frequencies, e.g., radio frequencies, components exhibit a useful impedance or range of impedance without being physically large. At audio frequencies, particularly at low audio frequencies, passive impedances such as capacitors and inductors, become physically large.
Another problem with all-pass filter circuits is that the phase shift is not adjustable and, in particular, is not readily adjustable over a wide range. For many circuits, it is possible to calculate component values as though such components were commercially available or at least real. Thus, a phase shift circuit may define a range resistance, preferably a linear change, of two orders of magnitude. Alternatively, one may calculate a value of capacitance that is either unobtainable in any form or impossible to produce in an integrated circuit.
Varying the value of a resistor or a capacitor can be done mechanically but is preferably done electrically. Absent electronic control, a circuit cannot be put to practical use in any sort of control loop. Thus, what is desired is an impedance that can be varied over a wide range electronically.
Impedance multipliers are known in the art but these are typically too complicated and costly. A relatively simple circuit is disclosed in U.S. Pat. No. 5,652,537 (Fleeman) and in a book by the same author. One circuit from the patent is reproduced in FIG. 2 herein and described below. The circuits disclosed in these publications are curiously described as having "active negative feedback" even though the inverting input of an operational amplifier is not used.
As recognized in the Fleeman patent, an FET is highly non-linear, variable resistance because variations in V.sub.ds on the FET can create distortions in the response curves of the FET. The patent proposes improving linearity by reducing V.sub.ds to less than .+-.50 millivolts, which may make the circuit operative in a technical sense but of little practical interest. The Fleeman patent discloses an FET only in a voltage divider. By inference, using an FET as the multiplied impedance would multiply the non-linearity as well and should not be done.
In view of the foregoing, it is therefore an object of the invention to provide a phase shift circuit that can provide an adjustable phase shift over a wide range of frequencies.
A further object of the invention is to provide a phase shift circuit that can operate on input signals of five volts or more.
Another object of the invention is to provide a phase shift circuit that can substantially be implemented as an integrated circuit.
A further object of the invention is to provide a phase shift circuit that multiplies the resistance of an FET to obtain a frequency range of four orders of magnitude or more.
Another object of the invention is to provide a phase shift circuit that employs an RC phase shift circuit in which the resistor is a FET.
A further object of the invention is to provide a phase shift circuit that combines a filter with an impedance multiplier.